A plate heat exchanger exchanges heat between two or more fluids. In most plate heat exchangers a number of stacked plate elements separate the fluids, each plate element having a central heat transferring part and a surrounding edge part. In some cases particular care must be taken to avoid one heat exchanging fluid from leaking into the flow way of another heat exchanging fluid. This is, e.g., the case in heat exchangers which are used for heating or cooling potable fluids using non-potable fluids, in heat exchangers used for processing critical fluids, and in heat exchangers in which mixing of the two fluids would result in undesired chemical reactions. In these cases a heat exchanger of the double wall type is normally used. In double wall heat exchangers the plate elements separating the heat exchanging fluids each comprises two plates which are joined together. For brazed heat exchanger brazing of some areas must be avoided.
In order to be able to detect a leak in one of the plates, the plates are often joined together in such a manner that leaking fluid is allowed to flow between the plates towards the edge portion of the plate element, e.g. to a location where it can be detected. Fast detection of a leak requires that the plates are arranged with a sufficient spacing to allow leaking fluid to flow easily towards the detecting position. On the other hand, in order to provide sufficient efficiency in heat transfer between the heat exchanging fluids, it is desirable to arrange the plates as close to each other as possible. Accordingly, various attempts have previously been done to design double wall heat exchangers taking these two requirements into consideration.
U.S. Pat. No. 5,291,945 discloses one example of a double wall heat exchanger comprising a number of plate elements defining flow spaces between them. Each plate element comprises two nested plates which are pressed substantially to the same shape and which closely abut against each other but still admit a heat exchanging fluid leaking through a hole in one of the plates to be conducted between the plates to the edge portion of the plate element. A disadvantage of this heat exchanger is that the time elapsing from a leak occurs until leaking fluid is detected at the edge portion is relatively long due to the plates abutting closely against each other.
U.S. Pat. No. 6,662,862 discloses another example of a heat exchanger in which adjacent plates form a double wall plate unit. Ridges and valleys formed in one plate are arranged adapted to and in near contact with corresponding ridges and valleys of the other plate, except at certain points which are adapted to be arranged in contact with an adjacent double wall plate unit. At these points the plates of the double wall plate unit are arranged with a distance there between in order to avoid unwanted brazing material between the plates of the double wall plate unit, since this would introduce the risk of blocking the flow path formed between the plates, thereby preventing a possible leak from being detected.